Electroactive polymer actuator device and driving method

ABSTRACT

An actuator device has an electroactive polymer actuator ( 35 ) and an integrated piezoelectric transformer ( 30 ). At least the secondary side ( 34 ) of the transformer shares a piezoelectric electroactive polymer layer ( 36 ) with the electroactive polymer actuator, so that lower external voltages can be applied to the device. A diode ( 46 ) is connected between the secondary side of the transformer and the electroactive polymer actuator.

FIELD OF THE INVENTION

This invention relates to electroactive polymer actuators.

BACKGROUND OF THE INVENTION

Electroactive polymers (EAPs) are an emerging class of materials withinthe field of electrically responsive materials. EAPs can work as sensorsor actuators and can easily be manufactured into various shapes allowingeasy integration into a large variety of systems.

Materials have been developed with characteristics such as actuationstress and strain which have improved significantly over the last tenyears. Technology risks have been reduced to acceptable levels forproduct development so that EAPs are commercially and technicallybecoming of increasing interest. Advantages of EAPs include low power,small form factor, flexibility, noiseless operation, accuracy, thepossibility of high resolution, fast response times, and cyclicactuation.

The improved performance and particular advantages of EAP materials giverise to applicability to new applications.

An EAP device can be used in any application in which a small amount ofmovement of a component or feature is desired, based on electricactuation. Similarly, the technology can be used for sensing smallmovements. This invention relates in particular to actuators.

The use of EAPs in actuator devices enables functions which were notpossible before, or offers a big advantage over common actuatorsolutions, due to the combination of a relatively large deformation andforce in a small volume or thin form factor, compared to commonactuators. EAPs actuators also give noiseless operation, accurateelectronic control, fast response, and a large range of possibleactuation frequencies, such as 0-1 MHz, most typically below 20 kHz.

Devices using electroactive polymers can be subdivided into field-drivenand ionic-driven materials.

Examples of field-driven EAPs include Piezoelectric polymers,Electrostrictive polymers (such as PVDF based relaxor polymers) andDielectric Elastomers. Other examples include Electrostrictive Graftpolymers, Electrostrictive paper, Electrets, ElectroviscoelasticElastomers and Liquid Crystal Elastomers.

Examples of ionic-driven EAPs are conjugated/conducting polymers, IonicPolymer Metal Composites (IPMC) and carbon nanotubes (CNTs). Otherexamples include ionic polymer gels.

This invention relates in particular to actuator devices whichincorporate a field-driven EAP material. These devices are actuated byan electric field through direct electromechanical coupling. Theyrequire high fields (volts per meter) but low currents due to theircapacitive nature. Polymer layers are usually thin to keep the drivingvoltage as low as possible.

A first notable subclass of field driven EAPs are Piezoelectric andElectrostrictive polymers. While the electromechanical performance oftraditional piezoelectric polymers is limited, a breakthrough inimproving this performance has led to PVDF relaxor polymers, which showspontaneous electric polarization (field driven alignment). Thesematerials can be pre-strained for improved performance in the straineddirection (pre-strain leads to better molecular alignment). Normally,metal electrodes are used since strains usually are in the moderateregime (1-5%). Other types of electrodes (such as conducting polymers,carbon black based oils, gels or elastomers, etc.) can also be used. Theelectrodes can be continuous, or segmented.

Another subclass of interest of field-driven EAPs is that of DielectricElastomers. A thin film of this material may be sandwiched betweencompliant electrodes, forming a parallel plate capacitor. In the case ofdielectric elastomers, the Maxwell stress induced by the appliedelectric field results in a stress on the film, causing it to contractin thickness and expand in area. Strain performance is typicallyenlarged by pre-straining the elastomer (requiring a frame to hold thepre-strain). Strains can be considerable (10-300%). This also constrainsthe type of electrodes that can be used: for low and moderate strains,metal electrodes and conducting polymer electrodes can be considered,for the high-strain regime, carbon black based oils, gels or elastomersare typically used. The electrodes can be continuous, or segmented.

FIGS. 1 and 2 show two possible operating modes for an EAP device.

The device comprises an electroactive polymer layer 14 sandwichedbetween electrodes 10, 12 on opposite sides of the electroactive polymerlayer 14.

FIG. 1 shows a device which is not clamped. A voltage is used to causethe electroactive polymer layer to expand in all directions as shown.

FIG. 2 shows a device which is designed so that the expansion arisesonly in one direction. The device is supported by a carrier layer 16. Avoltage is used to cause the electroactive polymer layer to curve orbow.

The nature of this movement for example arises from the interactionbetween the active layer which expands when actuated, and the passivecarrier layer. To obtain the asymmetric curving around an axis as shown,molecular orientation (film stretching) may for example be applied,forcing the movement in one direction.

The expansion in one direction may result from the asymmetry in the EAPpolymer, or it may result from asymmetry in the properties of thecarrier layer, or a combination of both.

US 2007/216735 discloses an ink-jet head which uses a piezoelectricactuator to eject ink. A piezoelectric transformer is integrated withthe actuator.

SUMMARY OF THE INVENTION

A problem with field driven electroactive polymers is the rather highoperation voltages that are required, as mentioned above, to achievehigh electric field strengths in the devices to realize desireddeflections. Electronic driving circuits are used to generate these highvoltages. Driving voltage amplitudes of up to lkV are required for theseEAPs, so that high voltage devices need to be used and implemented,increasing the cost (and size) of the electronics enormously. Thedriving electronics and the EAP actuator are generally locally separatedfrom each other resulting in the usage of high voltage feed-wiresbetween the driver and the actuator. This however may be a safety issuefor example in the case of broken wires and also results in high,unwanted (or even not-allowed) electric and magnetic fields as well aselectromagnetic radiation, which may cause compliance certificationissues or even could harm users.

There is therefore a need for an EAP actuator design which addressesthese issues.

It is an object of the invention to fulfil this need at least partially.The invention is defined by the independent claims and the dependentclaims provide advantageous embodiments.

Examples in accordance with an aspect of the invention provide anactuator device comprising:

an electroactive polymer actuator; and

a piezoelectric transformer having a primary side and a secondary side,

wherein the actuator device comprises a piezoelectric electroactivepolymer layer comprising a first portion and a second portion,

wherein the first portion of the electroactive polymer layer forms partof the secondary side of the piezoelectric transformer and the secondportion of the electroactive polymer layer forms part of theelectroactive polymer actuator.

In this actuator device, a part of an EAP actuator, which does not formpart of the actuation (output) portion of the device, is used to form atleast part of a piezoelectric transformer. In this way, the requiredhigh voltage drive signal may be generated locally using an integratedhigh voltage transformer. This has cost advantages, avoids the need forhigh voltage feed wires and also relaxes the electromagnetic radiationeffects.

A diode arrangement is electrically connected between the secondary sideof the piezoelectric transformer and the electroactive polymer actuator.This functions as an integrated rectifier, and may also be used toprovide protection for high voltage amplitudes with an undesiredpolarity.

In one set of examples, the first portion of the piezoelectricelectroactive polymer layer also forms part of the primary side of thepiezoelectric transformer. In this way, a shared EAP layer is used inthe primary transformer side, secondary transfer side and actuator partsof the overall device. This provides a low cost solution with maximumintegration.

The first portion of the piezoelectric electroactive polymer layer mayhave a first molecular orientation at the primary side and a second,different, molecular orientation at the secondary side. In this way,although a shared layer is used, the properties may be tailored toachieve improved performance of the transformer.

For example, the first molecular orientation may be in a longitudinaldirection extending between the piezoelectric transformer and theelectroactive polymer actuator, and the second molecular orientation maybe perpendicular to the plane of the piezoelectric electroactive polymerlayer (i.e. vertical).

In another set of examples, the primary side and secondary side of thepiezoelectric transformer are formed from different electroactivepolymer materials. There is still a shared layer between the secondaryside and the actuator. However, the transformer performance may beimproved by using different materials at the primary side and thesecondary side. The primary side provides electrical to mechanicalconversion, and the secondary side provides mechanical to electricalconversion. In this device, different EAP technology types may be used.

The electroactive polymer material of the primary side of thepiezoelectric transformer for example comprises a pre-straineddielectric elastomer.

In all examples, the primary side of the piezoelectric transformer maycomprise a multilayer stack. This enables a desired transformer ratio tobe obtained. The electroactive polymer actuator may also be formed as amultilayer stack.

An isolation region may be provided in the piezoelectric electroactivepolymer layer between the secondary side of the piezoelectrictransformer and the electroactive polymer actuator. This may be used toprovide mechanical decoupling between the transformer and the actuator.In particular, the damping of the transformer function by the actuatoroperation may be reduced.

The isolation region may comprise one or more openings in thepiezoelectric electroactive polymer layer and/or an an additionalinterface material.

The piezoelectric transformer may comprise a set of coplanar transformerelements. In this way, each transformer element may have a desired ratioof thickness to linear dimension (in the plane of the layer). Inparticular, if a low thickness is desired for miniaturization purposes,the use of multiple (smaller area) coplanar transformer elementsmaintains a desired ratio of thickness to in-plane size. This helpsmaintain a high efficiency of the transformer.

The transformer elements may be electrically connected in parallel, withtheir inputs in parallel and their outputs in parallel, or in series, oras a combination of parallel and series transformer elements.

The piezoelectric transformer may be flexible. The piezoelectricelectroactive polymer layer for example comprises polyvinylidenefluoride (PVDF) or polyvinylidene fluoride-trifluoroethylene(PVDF-TrFE).

Examples in accordance with another aspect of the invention provide amethod of driving an electroactive polymer actuator, comprising:

applying a drive signal to a piezoelectric transformer having a primaryside and a secondary side;

coupling the secondary side of the piezoelectric transformer to theelectroactive polymer actuator using a common a piezoelectricelectroactive polymer layer, such that a first portion of theelectroactive polymer layer forms part of the secondary side of thepiezoelectric transformer and a second portion of the electroactivepolymer layer forms part of the electroactive polymer actuator;

coupling a diode arrangement between the secondary side (34) of thepiezoelectric transformer and the electroactive polymer actuator (35);and

driving the electroactive polymer actuator using the output from thepiezoelectric transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a known electroactive polymer device which is not clamped;

FIG. 2 shows a known electroactive polymer device which is constrainedby a backing layer;

FIG. 3 shows a first example of an integrated EAP actuator andtransformer;

FIG. 4 shows a second example of an integrated EAP actuator andtransformer;

FIG. 5 shows a third example of an integrated EAP actuator andtransformer; and

FIG. 6 shows three possible transformer circuits.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an actuator device which has an electroactivepolymer actuator and an integrated piezoelectric transformer. At leastthe secondary side of the transformer shares a piezoelectricelectroactive polymer layer with the electroactive polymer actuator.This provides a device with an integrated transformer so that lowerexternal voltages can be applied to the device.

In general terms, field driven electroactive polymer (EAP) actuatorsconsist of an electrically isolating material, embedded between twoelectrically conducting electrodes. As a function of an applied voltage,the electric field between the electrodes causes a mechanicaldeformation of the EAP. As explained above, by using additionalmaterials with a different extension coefficient (e.g. stiffness) to theEAP layer, the mechanical deformation can be directed in certaindirections. By implementing multilayer technology, the field strengthbetween the electrodes can be increased and hence lower voltageamplitudes are required to operate the EAP actuator, or highermechanical deformations can be realized. This basic configuration isvery similar to an electric multilayer capacitor.

A piezoelectric transformer (also known as a dielectric transformer) isanother known passive device which can make use of both multilayer andmonolayer technologies. Depending on the configuration, very highvoltages can be generated, for example in the range of several kilovoltsat very high efficiencies, for example more than 90%. Such transformersmay be formed as flat devices for use in flat applications, where notenough space is available for conventional magnetic transformers.

A drawback of known piezoelectric transformers is their stiffness andbrittleness caused by the typically used ceramic material. They aretypically embedded in flexible (e.g. silicone) holders for this reason.In addition to a mechanical protection function, this soft and flexibleembedded design enables the transformer to vibrate mechanically. Thisreduces mechanical damping, which would decrease the efficiency. Theapplied electrical field at the primary side (based on an applied inputvoltage) causes the primary side to deform. The primary and secondaryparts are mechanically connected as a single integrated device, so themechanical primary deformation is coupled to the secondary side of thedevice. The mechanical deformation generates an electrical field again,causing a voltage drop over a connected load.

From an operational point of view, different device vibration modes canbe considered. Most common are transversal, longitudinal, thickness,planar, or radial modes as well as combination thereof. Most commonly,Rosen-type transformers use a combination of transversal andlongitudinal vibration modes. Devices are also known which rely onthickness and radial vibration modes.

The voltage transformation ratio from the primary side to the secondaryside basically depends on the form factor, the length (l) and the height(d) (i.e. thickness) of the component as well as the number of internallayers of the primary side (n) and secondary side. The performancedepends on several material and design parameters as will be known topeople skilled in the art. Further, from an electrical point of view,the output voltage also shows a dependency on the frequency of operation(ω=2πf) and the load conditions.

The invention makes use of a piezoelectric EAP layer to form at leastpart of the electromechanical conversion layer of the transformer. Thesame layer then forms at least part of the transformer (the secondaryside) as well as the EAP actuator.

FIG. 3 shows a first example of an actuator device. The device is shownin cross section in a vertical plane. It comprises a single integrateddevice which combines a piezoelectric transformer 30, having a primaryside 32 and a secondary side 34, with an electroactive polymer actuator35.

An AC voltage source 31 is at the input to the transformer.

Both the operation frequency and the conversion ratio of the transformerdepend on the transformer design and can be tuned. Typical values for acommercially available piezoelectric transformer are a frequency of 85kHz and conversion ratio of 50.

Generally, the frequency may be in the range of a few kHz up to a fewhundred kHz, for example 10 kHz to 100 kHz. The voltage transformationratio is for example in the range of 1 (only providing isolation andimpedance transformation) to 1000, for example in the range 10 to 100.

In the example shown, there is a single electroactive polymer layer 36which comprises a first portion 36 a which extends across the primaryand secondary sides of the transformer 30, and a second portion 36 bwhich extends across the actuator 35.

Each side of the transformer comprises a portion of the piezoelectricelectroactive polymer layer 36 sandwiched between upper and lowerelectrodes. The primary side 32 has an upper electrode 38 and a lowerelectrode 39, and the secondary side 34 has an upper electrode 40 and alower electrode 41. The electroactive polymer actuator 35 has an upperelectrode 42 and a lower electrode 43.

The first portion 36 a of the electroactive polymer layer thus formspart of the piezoelectric transformer and the second portion 36 b of theelectroactive polymer layer forms part of the electroactive polymeractuator 35.

A carrier layer 44 is shown at the EAP actuator 35, for controlling orconstraining the movement of the EAP actuator 35. It may of courseextend across the full device.

The piezoelectric transformer is thus implemented using thepiezoelectric EAP layer 36. The EAP layer 36 needs to be piezoelectricat least on the secondary side 34 of the transformer to provide therequired mechanical to electrical conversion. Thus, the example of FIG.3 uses a single polymer material which is piezoelectric. By way ofexample, PVDF or PVDF-TrFE may be used, or other piezoelectric polymers.

An advantage of the use of an EAP material in the transformer, inaddition to the integration with the actuator, is that the basicdrawback of a ceramic piezoelectric transformer, which is itsbrittleness, is addressed. The possible uses are thus extended and thepossible shapes are less limited (for example ceramic piezoelectrictransformers are generally limited to rectangular bars and generallywith a height of at least 2 mm). Thinner transformers may be fabricatedbut then they are even more brittle. The EAP-based transformer mayinstead be a flexible lightweight device, which can easily be designedto any arbitrary shape, such as a curved design. It may also have aheight of only a few hundred micrometres.

The desired dimensions (in plan view) will depend on several parameterand design implementations, such as the material properties and thenumber of layers. Very important is also the power/energy transfercapability, which correlates to the rate of charge-transfer (incombination with the voltage). This again relates to the chosen materialproperties and volume. The height (i.e. thickness) of the component aswell as the width and length is also important.

EAPs are low power components, and accordingly the energy transfer isintrinsically low. For example, a standard EAP operated by a steeprectangular on-pulse of 200 V requires a peak power of about only 230mW. After full activation to steady state operation, the required poweris in the low mW regime (to compensate for intrinsic losses).

An indication of a suitable size range may be obtained based on therequired performance. For example, to reach a delivered peak power anenergy of 2.1 mWs needs to be pushed into the EAP (for example over atime period of around 20 ms). Linearizing this energy yields an averageenergy of ½*2.1 mWs=1.05 mWs which needs to be delivered by thetransformer.

Based on the reference ‘Biomedical Applications of EAPs’; F. Carpi, E.Smelal; Wiley; p. 327, the energy density of P(VDF-TrFE-CFE), which isbe a suitable candidate for a transformer material, is known to be 1.22Ws/cm³. For comparison, the energy density of standard ceramic PZT isonly 0.1 Ws/cm³.

Thus, in this example, a volume of 1.05 mWs*1 cm³/1.22 Ws=0.0009 cm³ isrequired, i.e. approximately 1 mm³.

For the lateral dimensions, of one winding part of the transformer mayoccupy 10 mm×10 mm. This yields a height in the range of 10 μm(excluding electrodes). Thus the whole transformer (both primary andsecondary sides) has dimensions of the order of 20 mm×10 mm×0.01 mm.

This is simply an illustration to show that the required transformersize is in the range of the size of the active part of the EAP actuator(even when taking a low thickness of 10 μm) and hence does not increasethe height of such an actuator.

It is desirable that the actuation function does not influence theoperation of the transformer, for example the transformation ratio. Themovement of the EAP actuator should therefore be isolated from thetransformer. This may be ensured by defining a mechanically non-activepart of the EAP layer 36 between the transformer and the EAP actuator,for example by having an area without any covering material layer,whereas such a covering layer (with different mechanical expansioncoefficients) is used in the EAP actuator. This is shown in FIG. 3,where the carrier layer 44 is only used in the EAP actuator 35. However,the mechanical deformation caused by the actuation typically provides areduced layer thickness, which in turn (beneficially) increases thevoltage transformation ratio of the EAP-based transformer.

These issues can be taken into account in the design of the overalldevice.

An additional layer may also be provided on one or both sides of thedevice to improve the mechanical coupling between primary and secondarysides of the transformer. An additional stiffer layer or layers may beused to provide a preferred direction of the mechanical deformation.

To reduce the additional damping of the transformer function, caused bythis stiffer layer, the inner sides at the interface between theadditional stiffer layer and the transformer may be prepared to reducefriction, for example using low friction surfaces or a friction reducingfluid such as oil.

For the primary side of the transformer, a high deformation is desiredwith sufficient mechanical energy to transfer to the secondary side. Forthe secondary side, a high piezoelectric effect is desirable. There areseveral ways to improve the performance compared to a single uniform EAPlayer across the transformer, such as a PVDF-relaxor polymer.

A first modification is to use controlled molecular orientation in theprimary side and secondary sides. A chosen molecular orientation willincrease the electromechanical coupling (k). For example, the theprimary side may have a molecular orientation in the longitudinal(horizontal) direction, resulting in more mechanical energy beingprovided towards the secondary side. The secondary side may have amolecular orientation in the vertical direction. The higher couplingcoefficient value will result in a greater amount of conversion toelectrical energy.

A second modification is to use two different materials, one for theprimary side and one for the secondary side. The primary side forexample may use be a pre-strained dielectric elastomer (for exampleacrylics), having a high mechanical energy and a high couplingcoefficient, or a PVDF-relaxor polymer. The design is chosen to optimizethe conversion from electrical to mechanical energy.

The use of different materials may be combined with the selection ofdifferent molecular orientations.

The secondary side may then comprise a material which is brought intoresonance by the primary side, based on a relatively high piezoelectriceffect, for example a piezoelectric polymer (film), piezoelectriccopolymer or a PVDF relaxor polymer with a low CFE/CTFE content.Ferro-electrets may be used in the secondary side for their highpiezoelectric effect.

The primary side 32 of the transformer 30 may use multilayer stack 48 asshown in FIG. 4, mechanically coupled to a single layer secondary side34. The multilayer structure 46 at the primary side means than very highfield strengths are applied to the inner layers, causing largemechanical deformations of both the primary and the mechanically coupledsecondary side.

The multilayer stack 48 comprises thinner EAP layers, each providedbetween a pair of electrode layers, with the layers stacked withalternating polarity electrodes. Thus, only one pair of drive signals isneeded, and interleave layered comb electrodes provide driving of themultiple layers. The deformation at the secondary side results in thegeneration of an even higher electrical field strength across the outputterminals of the device and accordingly a correlated electrical voltage.

The transformer 30 does not require any stiff layers and can be keptfully flexible. A low voltage is applied to the primary side of thetransformer which is then up-converted and supplied to the active EAPactuator 35. This setup is especially efficient for miniature actuators.

If a dc operating voltage is required (instead of a pulse-wise operationmode), a rectifying diode may be added between the secondary side of thetransformer and EAP actuator.

FIG. 4 (and also FIG. 3) also shows the secondary side output rectifiedby a simple diode 46 which connects between the electrode 40 of thesecondary side and the electrode 42 of the EAP actuator. In addition tothe rectifying function, the diode provides protection of the actuatoragainst unwanted high voltage amplitudes with the opposite polarity tothe desired drive polarity. A discharging terminal may then be added tothe EAP actuator (not shown).

There are different possible operation modes of the actuator, and thediode may be needed for some and not for others.

In a pulsed operation mode of the actuator, the actuator may be used togenerate short (non-static) mechanical deflections. In this case thediode is not needed between the secondary side of the transformer andthe actuator. High frequency pulses may be used to maintain themechanical deflection, although this can introduce losses and result inheating and temperature drift.

In a steady state mode of operation of the actuator, the actuator may bemechanically deflected and required to maintain its state for a period.If only a single pulse is applied, the mechanical deflection will changeover time. In this case, a DC driving voltage is used and the diode isemployed. The transformer section works only with a non-DC (sinusoidalor pulsed waveform) input. Thus, the non-DC voltage at the secondaryside is rectified by the diode. The self-capacitance of the actuatorwill also smooth the signal.

In order to increase the mechanical deformation of the EAP actuator 35,a multilayer technique may also be used within the EAP actuator 35, ashas been explained in connection with the primary side 32 of thetransformer. This increases the electric field strength and thus causesgreater deflections.

The transformer may be (partly) polarized to improve the voltagetransformation performance. Any polycrystalline ceramic is composed of amultitude of randomly oriented crystals (dipoles) and the bulkproperties are the sum of the properties of these crystallites. In themanufacturing of piezoelectric ceramics, a suitable ferroelectricmaterial is first fabricated into a desired shape and electrodes areapplied. The piezoelectric element is then heated to the Curietemperature: the temperature above which the spontaneous polarizationand piezoelectric effect cease to exist. The heating is performed in thepresence of a strong DC field. This polarizes the ceramic (i.e. alignsthe molecular dipoles of the ceramic in the direction of an appliedfield). The polarization field remains frozen in place when thetemperature is reduced below the Curie point and the field is removed.The greater the number of domains aligned, the greater the piezoelectriceffect.

By poling the primary and secondary side accordingly (wherein the idealdirection of the dipoles depends on the chosen transformer principle) ofa piezoelectric transformer the efficiency can be increased very well.

The mechanical coupling between the transformer and the EAP actuator bythe shared EAP layer means that the actuation can affect the transformerfunction, as mentioned above.

FIG. 5 shows one approach for reducing the mechanical coupling betweenthe transformer 30 and the EAP actuator 35. It shows a top view (with nodiode). The connection between the secondary side 34 of the transformerand the EAP actuator area includes an isolation region 50. In theexample shown, the isolation region comprises a set of vertical openings52 which reduce the mechanical damping effect by the EAP actuator on thetransformer. The openings do not need to be vertical. Indeed, anyweakening structure may be used to provide an isolation function.

Instead of using openings for the isolation region 50, the secondaryside of the transformer and the EAP actuator may be connected by anadditional interface in the form of a soft material, for reducing themechanical coupling. This material may for example fill even largeropenings in the EAP layer.

The greatest decoupling may be achieved by separating the EAP layer intotwo separate portions, connected by the electrode wires or layers, andassembled on a sub carrier (for example a flexible PCB or foil). In thiscase, the EAP layer is discontinuous, but it is still formed as parts ofa common overall layer. In other words, the same type of EAP layer 36 isformed for the transformer (secondary side) and the actuator.

One advantage of using EAPs as the transformer material is forintegration of the transformer and actuator functions as describedabove. To optimize the transformer performance, the ratio of thethickness to the length (or width) of the transformer must not be toolow. If it is too low the efficiency and voltage gain of the transformerwill decrease significantly. An advantage of EAP materials is that theycan be processed, for instance by printing methods or film drawing, intovery thin layers, typically thinner than ceramic materials. This has twoadvantages.

First, the number of internal layers of the primary side can beincreased for a given thickness of the secondary part, increasing thevoltage gain.

Second, the total thickness of the device can be decreased. This secondadvantage enabled miniaturization of the transformer as described above.However, the ratio of the thickness to the length (or width) of thetransformer must remain high enough. This can be achieved by dividingthe transformer into an array of transformer elements by patterning theelectrodes into an array of Rosen type transformers.

FIG. 6A shows two transformer elements 60 a, 60 b in parallel. Thevoltage source 31 connects to the inputs of both transformer elements inparallel, and they both connect to the output 62 a, 62 b in parallel. Aparallel connection provides a higher current transformer design.

FIG. 6B shows two transformer elements 60 a, 60 b in series, wherein theoutput of the first transformer element 60 a forms the input of thesecond transformer element 60 b, and the output of the secondtransformer element 60 b connects to the transformer output 62 a, 62 b.A series connection provides a higher voltage ratio transformer design.

FIG. 6C shows a combination of series and parallel connections, with twoparallel elements 60 a, 60 b and two series elements 60 c, 60 d.

The integrated device described above enables the required input voltageto the device to be reduced, improving the safety of the component andimproving electromagnetic performance. The device may be extremely flatand flexible without requiring any high voltage connections. Inaddition, lower drive frequencies may be used reducing losses in thedriving electronics in comparison to ceramic piezoelectric transformers.The transformer also introduces a galvanic isolation between the EAPactuator and the power supply.

Materials suitable for the EAP layer are known.

The primary side of the transformer may use any material having thedesired electrical to mechanical conversion function. The material forthe secondary side is a piezoelectric material to provide the requiredmechanical to electrical conversion.

Electro-active polymers include, but are not limited to, thesub-classes: piezoelectric polymers, electromechanical polymers, relaxorferroelectric polymers, electrostrictive polymers, dielectricelastomers, liquid crystal elastomers, conjugated polymers, IonicPolymer Metal Composites, ionic gels and polymer gels.

The sub-class electrostrictive polymers includes, but is not limited to:

Polyvinylidene fluoride (PVDF), Polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE), Polyvinylidenefluoride-trifluoroethylene-chlorofluoroethylene (PVDF-TrFE-CFE),Polyvinylidene fluoride-trifluoroethylene-chlorotrifluoroethylene)(PVDF-TrFE-CTFE), Polyvinylidene fluoride-hexafluoropropylene(PVDF-HFP), polyurethanes or blends thereof.

The sub-class dielectric elastomers includes, but is not limited to:

acrylates, polyurethanes, silicones.

The sub-class conjugated polymers includes, but is not limited to:

polypyrrole, poly-3,4-ethylenedioxythiophene, poly(p-phenylene sulfide),polyanilines.

Ionic devices may be based on ionic polymer-metal composites (IPMCs) orconjugated polymers. An ionic polymer-metal composite (IPMC) is asynthetic composite nanomaterial that displays artificial musclebehavior under an applied voltage or electric field.

In more detail, IPMCs are composed of an ionic polymer like Nafion orFlemion whose surfaces are chemically plated or physically coated withconductors such as platinum or gold, or carbon-based electrodes. Underan applied voltage, ion migration and redistribution due to the imposedvoltage across a strip of IPMCs result in a bending deformation. Thepolymer is a solvent swollen ion-exchange polymer membrane. The fieldcauses cations travel to cathode side together with water. This leads toreorganization of hydrophilic clusters and to polymer expansion. Strainin the cathode area leads to stress in rest of the polymer matrixresulting in bending towards the anode. Reversing the applied voltageinverts the bending.

If the plated electrodes are arranged in a non-symmetric configuration,the imposed voltage can induce all kinds of deformations such astwisting, rolling, torsioning, turning, and non-symmetric bendingdeformation.

In all of these examples, additional passive layers may be provided forinfluencing the electrical and/or mechanical behavior of the EAP layerin response to an applied electric field.

The EAP layer of each unit may be sandwiched between electrodes. Theelectrodes may be stretchable so that they follow the deformation of theEAP material layer. Materials suitable for the electrodes are alsoknown, and may for example be selected from the group consisting of thinmetal films, such as gold, copper, or aluminum or organic conductorssuch as carbon black, carbon nanotubes, graphene, poly-aniline (PANI),poly(3,4-ethylenedioxythiophene) (PEDOT), e.g.poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS).Metalized polyester films may also be used, such as metalizedpolyethylene terephthalate (PET), for example using an aluminum coating.

The invention can be applied in many EAP and photoactive polymerapplications, including examples where a passive matrix array ofactuators is of interest.

In many applications the main function of the product relies on the(local) manipulation of human tissue, or the actuation of tissuecontacting interfaces. In such applications EAP actuators for exampleprovide unique benefits mainly because of the small form factor, theflexibility and the high energy density. Hence EAP's and photoresponsivepolymers can be easily integrated in soft, 3D-shaped and/or miniatureproducts and interfaces. Examples of such applications are:

Skin cosmetic treatments such as skin actuation devices in the form of aresponsive polymer based skin patches which apply a constant or cyclicstretch to the skin in order to tension the skin or to reduce wrinkles;

Respiratory devices with a patient interface mask which has a responsivepolymer based active cushion or seal, to provide an alternating normalpressure to the skin which reduces or prevents facial red marks;

Electric shavers with an adaptive shaving head. The height of the skincontacting surfaces can be adjusted using responsive polymer actuatorsin order to influence the balance between closeness and irritation;

Oral cleaning devices such as an air floss with a dynamic nozzleactuator to improve the reach of the spray, especially in the spacesbetween the teeth. Alternatively, toothbrushes may be provided withactivated tufts;

Consumer electronics devices or touch panels which provide local hapticfeedback via an array of responsive polymer transducers which isintegrated in or near the user interface;

Catheters with a steerable tip to enable easy navigation in tortuousblood vessels;

Measurements of physiological human body parameters such as heart beat,SpO2 and blood pressure.

Another category of relevant application which benefits from suchactuators relates to the modification of light. Optical elements such aslenses, reflective surfaces, gratings etc. can be made adaptive by shapeor position adaptation using these actuators. Here one benefit of EAPsfor example is a lower power consumption.

The examples above are based on a transversal-mode operatingtransformer, with electrical isolation between the primary side and thesecondary side. However, other vibration mode based transformers arepossible as well. Internal electrodes may also be added to the secondaryside. The primary side may use a single layer or a multilayer structure.The primary and secondary sides may be referenced to a common potential.

One aspect of the invention relates to the use of a diode with thedevice. There are other aspects which do not require this feature.

The general common features of the invention are an actuator devicecomprising:

an electroactive polymer actuator (35); and

a piezoelectric transformer (30) having a primary side (32) and asecondary side (34),

wherein the actuator device comprises a piezoelectric electroactivepolymer layer (36) comprising a first portion (36 a) and a secondportion (36 b),

wherein the first portion (36 a) of the electroactive polymer layerforms part of the secondary side (34) of the piezoelectric transformerand the second portion (36 b) of the electroactive polymer layer formspart of the electroactive polymer actuator (35).

The use of a diode arrangement (46) is electrically connected betweenthe secondary side (34) of the piezoelectric transformer and theelectroactive polymer actuator (35) is one aspect.

A second aspect is forming the primary side (32) and secondary side (34)of the piezoelectric transformer from different electroactive polymermaterials.

A third aspect is that the transformer elements are electricallyconnected in parallel, with their inputs in parallel and their outputsin parallel, or in series, or as a combination of parallel and seriestransformer elements.

These second and third aspects do not require the use of a diode.

Note that these other aspects may be combined with other features, suchas the flexible design, the isolation region and the multistack design.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. An actuator device comprising: an electroactive polymer actuator; apiezoelectric transformer, wherein the piezoelectric transformer has aprimary side and a secondary side; a piezoelectric electroactive polymerlayer, wherein the piezoelectric electroactive polymer layer comprises afirst portion and a second portion; and a diode arrangement, wherein thefirst portion of the electroactive polymer layer forms a portion of thesecondary side and the second portion of the electroactive polymer layerforms a portion of the electroactive polymer actuator, wherein the diodearrangement is electrically connected between the secondary side and theelectroactive polymer actuator.
 2. The device as claimed in claim 1,wherein the primary side comprises a multilayer stack.
 3. The device asclaimed in claim 1, further comprising an isolation region, wherein theisolation region is disposed in the piezoelectric electroactive polymerlayer between the secondary side and the electroactive polymer actuator.4. The device as claimed in claim 3, wherein the isolation regioncomprises one or more openings in the piezoelectric electroactivepolymer layer.
 5. The device as claimed in claim 1, wherein thepiezoelectric transformer comprises a plurality of coplanar transformerelements.
 6. The device as claimed in claim 5, wherein the coplanartransformer elements are electrically connected in parallel, with theirinputs in parallel and their outputs in parallel.
 7. The device asclaimed in claim, wherein the piezoelectric transformer is flexible. 8.The device as claimed in claim 1, wherein the primary side is formedfrom a first electroactive polymer material wherein the secondary sideis formed from a second electroactive polymer material, wherein thefirst electroactive polymer material is different from the secondelectroactive polymer material.
 9. The device as claimed in claim 8,wherein the first electroactive polymer material comprises apre-strained dielectric elastomer.
 10. The device as claimed in claim 8,wherein the first electroactive polymer material has a first molecularorientation and the second electroactive polymer material has a secondmolecular orientation, wherein the first molecular orientation isdifferent from the second molecular orientation.
 11. The device asclaimed in claim 10, wherein the first molecular orientation is in alongitudinal direction extending between the piezoelectric transformerand the electroactive polymer actuator, and the second molecularorientation is perpendicular to the plane of the piezoelectricelectroactive polymer layer.
 12. The device as claimed in claim 1,wherein the piezoelectric electroactive polymer layer is selected fromthe group consisting of polyvinylidene fluoride (PVDF) or polyvinylidenefluoride-trifluoroethylene (PVDF-TrFE).
 13. A method of driving anelectroactive polymer actuator, comprising: applying a drive signal to apiezoelectric transformer, wherein the piezoelectric transformercomprises a primary side and a secondary side; coupling the secondaryside to the electroactive polymer actuator using a common apiezoelectric electroactive polymer layer, wherein a first portion ofthe electroactive polymer layer forms part of the secondary side and asecond portion of the electroactive polymer layer forms part of theelectroactive polymer actuator, coupling a diode arrangement between thesecondary side and the electroactive polymer actuator; and driving theelectroactive polymer actuator using the output from the piezoelectrictransformer.
 14. The method as claimed in claim 13, wherein the primaryside comprises a multilayer stack.
 15. The method as claimed in claim13, wherein the electroactive polymer actuator an isolation region,wherein the isolation region is disposed in the piezoelectricelectroactive polymer layer between the secondary side and theelectroactive polymer actuator.
 16. The method as claimed in claim 13,wherein the primary side is formed from a first electroactive polymermaterial, wherein the secondary side is formed from a secondelectroactive polymer material, wherein the first electroactive polymermaterial is different from the second electroactive polymer material.17. The device as claimed in claim 1, wherein the electroactive polymeractuator comprises a multilayer stack.
 18. The device as claimed inclaim 3, wherein the isolation region comprises an additional interfacematerial, wherein the additional interface material comprises one ormore openings.
 19. The device as claimed in claim 5, wherein thecoplanar transformer elements are electrically connected in series. 20.The device as claimed in claim 5, wherein the coplanar transformerelements are electrically connected in a combination of parallel andseries transformer elements.